Universal Terrestrial Radio Access Network (UTRAN) is a conceptual term that identifies that part of the network which consists of Radio Network Controllers (RNCs) and Node Bs. This communication network is commonly referred to as 3G. Evolved UTRAN (E-UTRAN) is an evolution of the 3G radio access network towards a high-data rate, low-latency and packet-optimised radio access network.
As stated in the document 3GPP TR 25.913 “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)” issued by the 3rd Partnership Project (3GPP), an evolved UMTS Terrestrial Radio Access Network (E-UTRAN) is supposed to support the mobility of user equipments (UE) within different E-UTRA carrier frequency layers and also to support the mobility between E-UTRAN and legacy technologies (such as UTRAN and GERAN). In order to realize such mobility scenarios, the UE should be able to perform downlink measurements on other frequency or other access technologies without significant performance degradation. These types of measurements (i.e. outside serving frequency layer or outside intra frequency) are performed during idle gaps. An idle gap implies a time period that the UE for instance can use for performing such measurements. When performing measurements, the UE does not receive or transmit any data or signaling information.
Idle gap patterns can be scheduled in the following ways:                Static idle gaps        Fully dynamic periodic gaps        Semi-dynamic periodic gaps        
In static idle gap patterns the network configures all the associated parameters at the time of measurement configuration. In this approach the gaps occur periodically. The periodic pattern is used throughout the measurement until modified by higher layer signaling. The UE will not be scheduled during the gaps instead it will perform the requested measurements.
Dynamic idle gaps are generally created through negotiation between UE and Node B.
In semi-dynamic gap allocation the gap pattern is initially assigned to the UE at the start of each measurement via RRC signaling in a Measurement Control message. But during the course of the measurement the gaps can be altered by sending short commands. The network can send information to the UE to indicate whether the UE shall consider the gap pattern to be on or off. When the gap pattern is off the UE may be scheduled during the gap pattern and so will not use the gap to perform measurements. On the other hand when the gap pattern is on, the UE will not be scheduled and so can perform measurements during the gaps.
Most user equipments have single receiver chain, which can operate on a single carrier frequency at a time. Therefore, in E-UTRAN the UE can perform neighbour cell measurements without idle gaps on the neighbour cells belonging to the serving carrier frequency provided these cells are well aligned in the frequency domain.
Generally, cell identification and handover measurements that are carried out on cells operating on a carrier frequency outside the serving frequency layer, i.e. on inter-frequency, require idle gaps. In E-UTRAN the following categories of handovers will require a UE to perform neighbour cell measurements with idle gaps:                Handovers to E-UTRA inter-frequency cells, Frequency Division Duplex (FDD);        Handovers to E-UTRA inter-frequency cells, Time Division Duplex (TDD);        Handovers to UTRA Frequency Division Duplex (FDD) cells;        Handovers to UTRA Time Division Duplex (TDD) cells;        Handovers to GSM Edge Radio Access Network (GERAN) cells;        Handovers to non 3GPP technologies, e.g. CDMA2000, Mobile Wimax, etc.        
During an idle gap the UE tunes its receiver to another E-UTRA carrier frequency or to a carrier frequency of another access technology (e.g. UTRA or GERAN) for performing the neighbour cell measurements and/or cell identification. The more general term measurements is used which implies neighbour cell measurements as well as cell identification. It should be noted that in order to be able to perform neighbour cell measurements, the UE should first identify the cell. The neighbour cell measurements are performed on identified neighbour cells. While performing such measurements the UE does not receive or transmit any data or signaling information on the serving E-UTRA carrier frequency.
It is likely that at least some sort of semi-dynamic idle gaps will be used in E-UTRAN. In a semi-dynamic idle gap pattern assignment one or more gap patterns are initially assigned (or pre-configured) to the UE at the time of measurement configuration via higher layer signalling such as radio resource control (RRC). During the course of measurement the gaps can be activated or deactivated by sending short and fast commands, typically “on”/“off”-signals, that are sent on a shared control channel (e.g. L1/L2 control) as proposed in the document 3GPP TR 25.814 “Physical layer aspect for Evolved Universal Terrestrial Radio Access (UTRA)”. The commands are sent just prior to the start of gaps. Even if static idle gaps are used, on/off commands can still be used for activating or deactivating an entire gap pattern, i.e. to start or stop the idle gap pattern. Irrespective of the gap pattern type, the on/off commands can also be used at the very beginning, or activation, of the pattern and for terminating the pattern when measurements are over. It should be noted that the conventional way of activating or deactivating the entire pattern is via Radio Resource Control (RRC) signalling, whereby the drawback is that it is slow and therefore may cause performance degradation.
The main advantage of the “on”/“off”-commands is to provide flexibility to the scheduler since it can schedule users during the gaps if resources are available, if the quality of service is to be met, and if there is sufficient traffic in the buffer. Another main advantage is that HARQ (Hybrid Automatic Repeat Request) initial transmission and especially retransmissions would not be delayed due to measurements during the gap.
As shown in FIG. 6, each pre-configured idle gap pattern is characterized by a start sub frame number (SSSFN), an end sub frame number (ESSFN), and an inter-gap length (IGL). The typical gap length can vary between 2 ms and 10 ms duration. The gap pattern can be configured for a limited duration but also for infinite amount of time, i.e. until the end of the session. During an idle gap the UE can perform downlink measurements unless forbidden by the network via shared control channel (L1/L2 control) as explained above. In case a measurement is not allowed, e.g. by a gap “off”-command, the UE expects to be scheduled for data transmission.
Shared controlchannel fieldInterpretationAction0Gap ONPerform measurement only1Gap OFFSkip measurement,receive/transmit data
The table illustrates an example of the connotation of gap commands and how gap commands can be interpreted. This, however, is purely a matter of definition; alternative approaches could define, for instance, the values 0 and 1 to be specified as gap “off” and gap “on”, respectively.
One particularly important issue in transmissions is the reliability. I.e. the entire decision process relies upon one or more simple commands, such as the “on”/“off”-signalling, up and down commands, etc. Thus, unreliable commands may cause actions to occur in reverse direction and in some cases may lead to unstable behaviour. Unreliabilities where a receiver cannot properly interpret the correct meaning of a received command generally occur due to bad radio conditions, low transmitted power level, poor coverage, high system load, etc. FIG. 7 illustrates an example in which an unreliable gap command leads to a missed HARQ transmission.
To ensure reliable operations, four sets of functionalities should be specified:                Reliability check;        Behaviour or action of UE and/or base station;        Indication or reporting of unreliability events to network;        Prevention of unreliability.        
The reliability check can be based either on some signal strength or quality thresholds. It can also be based on some bit error rate (BER) target value. This means a received command is regarded as unreliable in case the received signal quality or strength falls between the thresholds or if the BER is higher than the target.
The behaviour of UE or base station in response to unreliable command detection depends upon a particular functionality governed by the on/off command. This is described with the following examples from UTRAN:
Assuming for instance TPC commands (“on”/“off”-type signalling) as used in WCDMA for inner loop power control in a soft handover scenario. The UE is supposed to regard the UL TPC command (received on the downlink) as unreliable in case TPC command error rate exceeds a certain threshold, e.g. 30%. The immediate action or behaviour of the user equipment is to disregard such a command when combining TPC commands in soft handover from more than one radio link sets. Both reliability check and UE actions are generally mandated by a suitable test case as is the case with TPC combining.
Another example relates to the UE behaviour in case of unreliable scheduling grants in E-DCH transmission. An unreliable up- or down-command is treated as a hold command by the UE. This means that the UE in case of a detected unreliable grant will neither increase nor decrease its current transmitted rate. This well specified behaviour ensures that a UE shall not transmit with unnecessary higher power because this would otherwise increase the noise rise at the base station.
In order to eliminate the unreliable behaviour, the network needs to take some corrective actions (such as increasing power level, performing congestion control, doing handover, etc.), which is generally possible if the network is made aware of the ‘unreliable’ behaviour, preferably by reporting ‘successive unreliable occurrences’.
Reliability of the received commands can be improved by using redundant bits (e.g. sending 000 and 111 for 0 and 1 respectively). But this is not sufficient as this is almost always done (e.g., 2 or more bits per TPC command) but still UE behaviour needs to be specified since redundant bits may also become unreliable.